Method for forming a hardmask employing multiple independently formed layers of a pecvd material to reduce pinholes

ABSTRACT

A bi-layer BARC/hardmask structure includes a layer of amorphous carbon and two or more distinct and independently formed layers of a PECVD material such as SiON formed on the amorphous carbon layer. By independently forming several layers of PECVD material, at least some pinholes that are present in the lowermost PECVD layer are closed by upper PECVD layers and therefore do not extend through all of the PECVD layers. As a result the upper surface of the uppermost PECVD layer has a lower pinhole density than the lower PECVD layer. This reduces photoresist poisoning by dopant in the amorphous carbon layer, and etching of the amorphous carbon layer by photoresist stripping chemistry.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to fabrication of semiconductor devices, and inparticular, to bi-layer hardmasks that include a plasma-enhancedchemical vapor deposition (PECVD) layer such as PECVD silicon oxynitride(SiON).

2. Background Technology

Features of semiconductor devices such as gate lines are typicallypatterned using a bi-layer structure that serves as a bottomantireflective coating during photoresist patterning, and that furtherserves as a hardmask during patterning of an underlying patternablelayer. FIG. 1 shows a structure that may be employed in such processingto form a gate line of a MOSFET. As shown in FIG. 1, a semiconductorsubstrate 10 includes isolations 12 that define an area in which aMOSFET is to be formed. A conformal gate insulating layer 14 such assilicon oxide is formed over the substrate 10 and oxides 12. A gateconductive layer 16 such as polysilicon is deposited over the gateinsulating layer 14 and will be patterned to form a gate line. Formedover the gate conductive layer 16 is a bi-layer structure that serves asa bottom antireflective coating (BARC) and as a hardmask for patterningunderlying layers. The bi-layer structure includes an amorphous carbonlayer 18 and a PECVD SiON capping layer 20 having a thickness ofapproximately 260 angstroms. The amorphous carbon layer is doped withnitrogen to improve its etch selectivity with respect to the underlyingpolysilicon 16. A photoresist mask 22 is formed on the SiON cappinglayer 20. The photoresist mask 22 is used to pattern the SiON layer 20,which in turn is used as a hardmask to pattern the amorphous carbonlayer 18, which in turn is used as a hardmask to pattern a gate linefrom the polysilicon layer 16.

As critical dimensions of semiconductor devices shrink, the dimensionsof the structures used to pattern those devices are also reduced. In thecase of the bi-layer structure of FIG. 1, this is seen as a decrease inthe thicknesses of the SiON and amorphous carbon components of thebi-layer. At current dimensions, such thinning can produce detrimentaleffects. One source of detrimental effects is the presence of “pinholes”in the PECVD capping layer. Pinholes are believed to be formed byoutgassing from underlying layers during deposition of the PECVDmaterial. For example, when SiON is formed over an amorphous carbonlayer, residual hydrogen may be emitted from the amorphous carbon layer.These emissions cause localized non-uniformities in the PECVD depositionplasma, which result in reduced deposition of SiON in the vicinities ofthe non-uniformities. Pinholes that extend partly or entirely throughthe SiON layer may form at those locations.

Pinholes are a source of at least two problems. One of these problems isphotoresist poisoning. As shown in FIG. 1b, a pinhole 24 enablesdiffusion of nitrogen dopant from the amorphous carbon layer 18 into anoverlying photoresist layer 26, forming a region of poisoned photoresist28. Poisoned photoresist exhibits reduced response to conventionalphotoresist development chemistries, and as a result, unwantedphotoresist bodies may be left behind after development, causingundesired patterning of underlying layers during subsequent processing.

A second problem caused by pinholes is premature etching of theamorphous carbon layer during reworking of photoresist. During typicalprocessing, photoresist layers may be applied over a bi-layer hardmask,patterned, and removed several times. As shown in FIG. 1c, duringremoval of photoresist, the chemistry used to strip the photoresist maypass through a pinhole and contact the underlying amorphous carbonlayer, causing etching of a region 30 in the amorphous carbon. Thisresults in the formation of anomalous patterns in the amorphous carbonthat may be transferred to underlying layers during subsequentprocessing. Such etching has been found to occur even with pinholes thatdo not extend completely through the SiON layer, a phenomena known as“punch through.”

Accordingly, there is a need for improved semiconductor processingtechniques that reduce the detrimental effects of pinholes in PECVDmaterials.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the invention, a bi-layerBARC/hardmask structure includes a layer of amorphous carbon and two ormore distinct adjacent layers of a PECVD material such as SiON that areformed independently over the amorphous carbon. By independently formingtwo or more distinct layers of PECVD material, pinholes that are presentin the lowermost layer may be prevented from continuing to grow duringformation of the overlying layers. Thus by using several layers of PECVDmaterial, the pinhole density at the surface of the PECVD portion of thebi-layer is decreased.

In accordance with one embodiment of the invention, a semiconductordevice may be fabricated by forming an amorphous carbon layer over apatternable layer such as polysilicon, and then forming a multi-layerPECVD material over the amorphous carbon. The pinhole density of anupper surface of an upper layer of PECVD material is thus lower than apinhole density of a lower layer of the PECVD material. A photoresistmask may then be formed over the layers of PECVD material, and a patternof the photoresist mask may be transferred to the patternable layer. Byproviding multiple layers of PECVD material, pinhole density is reduced,and so photoresist poisoning and etching of the amorphous carbon layerby photoresist stripping chemistry is reduced.

The layers of PECVD material may be formed either in situ or ex situ.The PECVD material may comprise any of a variety of materials, includingsilicon oxynitride, silicon carbide, silicon oxide, SiCH, and SiCOH.More than two layers of the PECVD material may be used.

In accordance with another embodiment of the invention, a bi-layerhardmask structure may include multiple distinct layers of PECVDmaterial. Therefore, the bi-layer hardmask may be incorporated intostructure formed during fabrication of a semiconductor device, includinga substrate comprising a patternable layer, an amorphous carbon layerformed over the patternable layer, and the PECVD material. The PECVDmaterial may include at least a distinct lower layer and a distinctupper layer of PECVD material formed over the amorphous carbon layer,such that the pinhole density of an upper surface of the upper layer islower than the pinhole density of the lower layer. A photoresist maskmay be formed over the upper layer of PECVD material for transferring ofa pattern of the photoresist mask to the patternable layer. By providingmultiple layers of PECVD material, pinhole density is reduced, and sophotoresist poisoning and etching of the amorphous carbon layer byphotoresist stripping chemistry is reduced.

The PECVD material may comprise any of a variety of materials, includingsilicon oxynitride, silicon carbide, silicon oxide, SiCH, and SiCOH.More than two layers of the PECVD material may be used.

DESCRIPTION OF DRAWINGS

Embodiments of the invention are described in conjunction with thefollowing drawings, in which:

FIG. 1a shows a conventional bi-layer structure for patterning a featureof a semiconductor device;

FIGS. 1b and 1 c show detrimental effects caused by pinholes in a SiONlayer of the conventional bi-layer;

FIG. 2 shows a preferred embodiment in accordance with the invention;

FIG. 3 shows a section of the embodiment of FIG. 2; and

FIG. 4 shows a process flow encompassing the preferred embodiment andother alternative embodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2 shows a preferred embodiment in accordance with the invention. Asshown in FIG. 2, the structure is similar to the conventional structureof FIG. 1a, in that it includes a semiconductor substrate 10, isolations12, a gate insulating layer 14, a polysilicon gate conductive layer 16,a bi-layer including an amorphous carbon layer 18 and an overlying SiONportion, and a photoresist mask 22. However, in contrast to theconventional structure, the structure of the preferred embodimentemploys multiple independently formed layers 20 a, 20 b of SiON.

FIG. 3 shows the effect of multiple distinct layers of SiON on pinholegrowth. As shown in FIG. 3, pinholes 32, 34, 36 that are formed duringdeposition of a first layer of SiON 20 a extend through that layer ofSiON. However, by terminating the growth of the first SiON layer 20 a,and then subsequently beginning growth of another SiON layer 2 b, thepinholes 32, 34 are effectively closed. Therefore, while some pinholessuch as pinhole 36 may continue to grow, there is an over-all reductionin the number of pinholes that extend through both SiON layers, or thatextend sufficiently through to enable photoresist poisoning or amorphouscarbon etching to occur. Therefore the pinhole density at the surface ofthe PECVD material is reduced, providing fewer avenues for photoresiststripping chemistry to attack the underlying amorphous carbon 18, andfor dopant from the amorphous carbon 18 to poison overlying photoresist26.

The formation of multiple distinct layers may be performed using eitherin situ or ex situ processing. In the case of in situ processing, thelayers are formed without removing the substrate from the depositionchamber. After each individual layer is formed, gas flow is shut off toextinguish the deposition plasma. Gas flow is then reestablished and theplasma is re-ignited to commence deposition of the next layer. In thecase of ex situ processing, the substrate is removed from the depositionchamber after deposition of each SiON layer. In either case, the layersare distinct because they are independently formed by respectivediscontinuous deposition processes, and because some pinholes in lowerlayers are closed in the vicinity of the junctions between materialsformed by the respective independent deposition processes.

The pinhole density at the surface of materials formed using multipledistinct layers as described herein may be determined by opticalscattering using a KLA Tencor surface scan tool.

While the preferred embodiment of FIG. 2 employs two layers 20 a and 20b of SiON, in alternative embodiments a different number of layers maybe used. Moreover, the technique may be applied to other PECVD materialsthat may be used in a bi-layer hardmask. Examples of such PECVDmaterials include PECVD silicon oxide, PECVD silicon nitride, and otherPECVD films such as silicon carbide, SiCH and SICOH.

Further, while the patternable layer of the preferred embodiment is apolysilicon gate layer, in alternative embodiments other patternablematerials may be employed.

A process flow encompassing the preferred embodiment the aforementionedalternatives, and other alternative embodiments is illustrated in FIG.4. Initially a substrate is provided (40). The substrate comprises apatternable layer. An amorphous carbon layer is then formed over thepatternable layer (42). At least two distinct adjacent layers of a PECVDmaterial are then formed independently on the amorphous carbon layer(44). The at least two PECVD layers may be formed either in situ or exsitu. A photoresist mask is then formed over the at least two PECVDlayers (46), and the pattern of the photoresist mask is transferred tothe patternable layer (48), such as by sequentially etching the PECVDmaterial and the underlying materials using each material as a hardmaskfor patterning the underlying material.

Further processing may be performed after the processing shown in FIG.4, such as etching of layers beneath the patternable layer such as agate insulating layer, and formation of spacers, source and drainregions, source and drain contacts, and other types of structures.

It will be apparent to those having ordinary skill in the art that thetasks described in the above processes are not necessarily exclusive ofother tasks, but rather that further tasks may be incorporated into theabove processes in accordance with the particular structures to beformed. For example, intermediate processing tasks such as formation andremoval of passivation layers or protective layers between processingtasks, formation and removal of photoresist masks and other maskinglayers, formation of stress relief layers between other layers, as wellas other tasks, may be performed along with the tasks specificallydescribed above. Further, the process need not be performed on an entiresubstrate such as an entire wafer, but rather may be performedselectively on sections of the substrate. Thus, while the embodimentsillustrated in the figures and described above are presently preferred,it should be understood that these embodiments are offered by way ofexample only. The invention is not limited to a particular embodiment,but extends to various modifications, combinations, and permutationsthat fall within the scope and spirit of the appended claims.

What is claimed is:
 1. A method of fabricating a semiconductor devicecomprising: providing a substrate comprising a patternable layer;forming an amorphous carbon layer over the patternable layer; forming afirst layer of a plasma enhanced chemical vapor deposition (PECVD)material on the amorphous carbon layer; independently forming a secondlayer of the PECVD material directly on the first layer, such that apinhole density of an upper surface of the second layer is lower than apinhole density of the first layer; forming a photoresist mask over theuppermost layer of the PECVD material; and transferring a pattern of thephotoresist mask to the patternable layer.
 2. The method claimed inclaim 1, wherein the first layer of the PECVD material and the secondlayer of the PECVD material are independently formed in situ.
 3. Themethod claimed in claim 1, wherein the first layer of the PECVD materialand the second layer of the PECVD material are independently formed exsitu.
 4. The method claimed in claim 1, wherein the PECVD materialcomprises silicon oxynitride.
 5. The method claimed in claim 1, whereinthe PECVD material comprises silicon carbide.
 6. The method claimed inclaim 1, wherein the PECVD material comprises silicon oxide.
 7. Themethod claimed in claim 1, wherein the PECVD material comprises SiCH. 8.The method claimed in claim 1, wherein the PECVD material compriseSiCOH.
 9. The method claimed in claim 1, wherein the amorphous carbonlayer is doped with nitrogen to enhance the etch selectivity of theamorphous carbon layer with respect to an underlying polysiliconpatternable layer.
 10. The method claimed in claim 1, wherein at least athird layer of PECVD material is independently formed directly on thesecond layer of the PECVD material such that a pinhole density of anupper surface of the third layer is lower than a pinhole density of thesecond layer.
 11. The method claimed in claim 1, wherein the patternablelayer comprises a polysilicon gate conductive layer.